What is port 25 used for?
Port 25 is SMTP, the protocol mail servers use to hand messages to each other. When you send an email, your provider's server looks up the recipient domain's mail server and delivers the message to it on port 25.
Today port 25 is for relay between servers. Mail programs and devices that send mail are expected to use the submission port, 587, with sign-in. Because infected computers used to send spam straight out on port 25, some internet providers block outbound connections to it from customer networks, which is a classic reason why a scanner or an application cannot send email. Encryption on port 25 is opportunistic: servers upgrade the connection with STARTTLS when both sides support it.
Is port 25 TCP or UDP?
TCP. Mail delivery has to be reliable, so SMTP runs over TCP on port 25.
Is SMTP on port 25 encrypted?
Not by default. The session starts in clear text and is upgraded with the STARTTLS command only if both servers support it. Clients should submit mail on port 587, with authentication and TLS, instead of using 25.
Port 25 vs port 587: what is the difference?
Port 25 is for relay between mail servers. Port 587 is for submission: a mail client or device signs in and hands its message to its own provider. A printer or application that sends email should be pointed at 587. Port 587 explained
Which CompTIA exams list port 25?
CompTIA lists port 25 by number in both the Network+ and the A+ Core 1 objectives.
- Network+ N10-009: objective 1.4 (Networking Concepts), listed as “Simple Mail Transfer Protocol (SMTP), 25”.
- A+ Core 1 220-1201: objective 2.1 (Networking), listed as “25, Simple Mail Transfer Protocol (SMTP)”.
The quoted text is how the objectives document lists the port. The exam puts the port in a scenario, such as a firewall rule or a failed connection; it does not only ask for the number.
Check yourself: a practice question
Written by CertSprint for this page. It is not taken from any exam.
A new office scanner cannot send scans by email through the company's cloud mail provider. It is set to SMTP on port 25 with no sign-in. What should be changed first?
- Use port 587 with authentication and TLS
- Use port 110
- Switch the scanner to Telnet
- Give the scanner a static MAC address
Show the answer
A. Use port 587 with authentication and TLS. Providers accept mail from clients on the submission port with sign-in, and some networks block outbound 25. Port 110 is POP3, which receives mail.
Sources: IANA Service Name and Transport Protocol Port Number Registry: port 25; RFC 5321: Simple Mail Transfer Protocol; RFC 3207: SMTP Service Extension for Secure SMTP over Transport Layer Security; RFC 5068: Email Submission Operations: Access and Accountability Requirements. The port number and how each exam lists it come from CompTIA's exam objectives, linked from the study plans above. The explanation and the question are CertSprint's own, checked against these sources on 12 November 2026.